Improvement of the Anode Properties of Lithium-Ion Batteries for SiO x with a Third Element.

Abstract

Silicon oxide (SiOx) has been placed into practical use as an anode active material for next-generation Li-ion batteries because it has a higher theoretical capacity than graphite anodes. However, the synthesis method is typically vapor deposition, which is expensive, and the poor electron conductivity of SiOx restricts high performance. In this study, we prepared M/SiOx active materials consisting of SiOx and a third element (M = Al, B, Sn) using a low-cost mechanical milling (MM) method and investigated their electrode properties as Li-ion battery anodes. Also, the authors added a third element to improve the conductivity of the SiO2 matrix. Al, B, and Sn were selected as elements that do not form a compound with Si, exist as a simple substance, and can be dispersed in SiO2. As a result, we confirmed that SiOx has a nanostructure of nanocrystalline Si dispersed in an amorphous-like SiO2 matrix and that the third element M exists not in the nanocrystalline Si but in the SiO2 matrix. The electron conductivity of SiOx was improved by the addition of B and Sn. However, it was not improved by the addition of Al. This is because Al2O3 was formed in the insulator due to the oxidization of Al. The charge–discharge cycle tests revealed that the cycle life was improved from 170 cycles to 330 or 360 cycles with the addition of B or Sn, respectively. The improvement in electron conductivity is assumed to make it possible for SiO2 to react with Li ions more uniformly and form a structure that can avoid the concentration of stress due to the volume changes of Si, thereby suppressing the electrode disintegration.